THE president's ADDRESS. 



indisposition to a severe and fatal illness. Hence, in discussing 

 these parasites it is better to group them as lethal, or deadly, 

 and non-lethal, including under the latter term both harmless 

 parasites and those that produce ailments from which the host 

 recovers more or less easily. The distinction between lethal and 

 non-lethal parasites is not, however, in all cases absolute, since 

 a parasite which is ordinarily harmless or but slightly harmful 

 may exhibit lethal powers under certain conditions, especially when 

 the host is in an enfeebled state of vitality from other causes. 

 For instance, it is common to see animals living in captivity 

 succumb to the effects of parasites which are harmless to them 

 when living a natural healthy life in their wild state. The 

 weakened powers of resistance of the host enable the parasite 

 to flourish in an abnormal manner, and give it a capacity for 

 harmfulness which it does not possess naturally. Hard and fast 

 distinctions between different classes of parasites, from the 

 point of view of the effects on their hosts, cannot always be 

 drawn. 



The great diversity in the effects of Protozoan parasites is very 

 remarkable, and furnishes a most important subject for considera- 

 tion and investigation. It would seem, at first sight, as if the 

 presence of even a single parasite must cause some bad effect in 

 the body of the host, even if the disturbance be so slight as to 

 produce no appreciable symptoms of ill-health in the organism 

 as a whole. If so, then the greater the number of parasites in 

 the body of the host, or the larger the parasite itself, the greater 

 should be the derangement in the health of the host. Con- 

 sequently, the greater the powers of multiplication within the 

 body of the host possessed by any given parasite, the more 

 dangerous it might be expected to be. To a certain extent this 

 expectation is realised. Thus the Gregarines of the suborder 

 Etigregarinae, found commonly in the bodies of insects and 

 other arthropods, are amongst the most harmless of parasites, so 

 far as can be judged, and in these forms endogenous multiplica- 

 tion, as it is called, that is to say multiplication in the actively 

 parasitic phase' in the body of the host, does not take place. On 

 the other hand, the parasites of the closely allied order Coccidia 



